[mac] brew update Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

$brew update
error: Your local changes to the following files would be overwritten by merge:
README.md
Please, commit your changes or stash them before you can merge.
Aborting
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

解決方式

cd $(brew --prefix)
git reset --hard HEAD
brew update

2.

cd /usr/local && git reset --hard FETCH_HEAD

or if it fails

cd /usr/local && sudo git reset --hard FETCH_HEAD